Both of them are in god tier atm so both are good enough, but you need to think about team compositons before you pick any of them. In my opinion Draven is the most complex AD to play/master so if you have dilema which one to buy I'll recommand you Cait coz of better and easier learnning curve.

I think that if you learn to play draven you can carry every game in soloQ(exept wenn hole team feed and enemy team focus you hard).Cait have good damage but draven have more damage in small interval of time. So wenn you master draven you are GOD.